The present invention relates to acoustic projectors, especially projectors for use in low frequency military and civilian sonar systems, and in particular to underwater acoustic projectors having a flextensional outer wall surrounding an acoustic driver with that outer wall being securely attached to end caps which are mechanically coupled to each end of the driver.
Low frequency military and civilian sonar systems require compact, light weight, high power, efficient, wide bandwidth acoustic projectors whose performance is stable with depth and linear with drive voltage levels and which have a low manufacturing and maintenance cost.
Flextensional projectors are amongst the best ones presently available to meet the military and civilian sonar systems requirements, one of the most promising flextensional projectors being the barrel stave type. The barrel stave projector (BSP) is a compact, low frequency underwater sound source which has applications in low frequency active (LFA) sonar and in underwater communications. In one known BSP design, such as described in U.S. Pat. No. 4,922,470 by G. McMahon et al, a set of inwardly curved bars (staves) surround and enclose a stack of axially poled piezo-electric rings. The staves act like a mechanical transformer and help match the impedance of the transducer to the radiation impedance of the water. The staves are attached to octagonal end caps at each end of the stack by screws and epoxy glue. Axial motion of the stave ends is transformed to a larger radial motion of the stave midpoints. This increases the net volume velocity of the water, at the expense of the applied force, and is essential for radiating effectively at low frequencies.
This known BSP projector has slots between the staves which are required to reduce the hoop stiffness and achieve a useful transformer ratio. However, these slots must be waterproofed by a rubber membrane (boot) stretched tightly and glued with epoxy around the projector. This boot also provides effective corrosion protection for the aluminum staves.
Variants of this known BSP have been built to optimise light weight, wide bandwidth, low frequency, high power, and improved electroacoustic efficiency. Efficiency is an especially critical parameter for the high power versions of the BSP because the driver is well insulated from the water thermally. The boot""s relatively poor thermal conductivity contributes to the difficulty in cooling the BSP.
The inside surfaces of the (eight) staves of one type of BSPs are machined individually from bar stock on a numerically controlled (NC) milling machine. The staves are then mounted together on a fixture and the outside surfaces are turned on a tracer lathe. Holes drilled in flanges at each end of the staves allow the staves to be connected to end caps by screws and glue. These BSPs are, as a result, both relatively costly to manufacture and maintain.
One variant of a BSP is described by Robert J. Obara in U.S. Pat. No. 5,136,556 where the barrel staves are arranged in a circle and are attached to octagonal end caps but have elliptical cross-sections of varying eccentricity between top and bottom end caps. This arrangement provides a wider bandwidth than with a circular (octagonal) cross-section arrangement throughout the length of the projector as in U.S. Pat. No. 4,992,470. The eight staves in U.S. Pat. No. 5,136,556 are attached to the upper and lower octagonal end caps by screws.
Another type of BSP is described in European Patent Application 90313788.3 by George H. Cavanagh III. In one embodiment in the European Patent Application, twelve staves are fastened to a dodecagon (a twelve-sided regular polygon) by means of screws (line 6 to 10 in column 5).
The use of screws to attach staves to end caps weakens the ends of the staves and creates stresses around holes through which the screws are inserted, particularly during operation when the staves are flexed to project acoustic waves.
A one-piece flextensional shell projector is described by Christopher Purcell in U.S. Pat. No. 5,805,529. The surface of this projector is formed of a thin-walled one-piece inwardly concavely shaped shell containing corrugations running in the axial direction. This one-piece shell is slotless which eliminates the requirement for a boot. End flanges on the one-piece shell are attached to end caps at each end of an acoustic driver by screws but these weaken the flanges and create stresses during operation which can result in fatigue in the metal flanges around the holes through which the screws are inserted.
It is an object of the invention to provide an acoustic projector with a secure attachment of a flextensional outer wall of the projector to end caps that are located at each end of an acoustic driver, the flextensional outer wall surrounding the driver.
An acoustic projector, according to one embodiment of the present invention, comprises a pair of spaced apart end caps with an acoustic driver positioned between and mechanically coupled to the end caps, the driver having smaller cross-sectional dimensions than the end caps, a flextensional outer wall of the projector surrounds the driver and is secured to the end caps by rings heat shrunk over outer end flanges on the outer wall, which flanges are in contact with the end caps.